As a subclass of commercially available polymers, polyurethane elastomers have several properties whose advantages confer unique benefits on these products. Typically, polyurethanes show high abrasion resistance with high load bearing, excellent cut and tear resistance, high hardness, resistance to ozone degradation, yet are pourable and castable. Compared to metals, polyurethanes are lighter in weight, less noisy in use, show better wear and excellent corrosion resistance while being capable of cheap fabrication. Compared to other plastics, polyurethanes are non-brittle, much more resistant to abrasion, and exhibit good elastomeric memory. Polyurethanes find use in such diverse products as aircraft hitches, bushings, cams, gaskets, gravure rolls, star wheels, washers, scraper blades, impellers, gears, and drive wheels.
Part of the utility of polyurethanes derives from their enormous diversity of properties resulting from a relatively limited number of reactants. Typically, polyurethanes are prepared on site by curing urethane prepolymers, which are adducts of polyisocyanates and polyhydric compounds. A large class of such prepolymers are approximately 2:1 adducts of a diisocyanate, OCN--Y--NCO, and a diol, HO--Z--OH, whose resulting structure is OCN--Y--NHCO.sub.2 --Z--O.sub.2 CNH--Y--NCO. Although Y is susceptible of great variety, usually being a divalent alkyl, cyclohexyl, or aromatic radical, in fact the most available prepolymers are made from toluene-2,4-diisocyanate (TDI) or methylene-4,4'-diphenyldiisocyanate (MDI). The diols used display a greater range of variety; Z may be a divalent alkyl radical (i.e., an alkylene group), and the diols frequently are ethers or esters which are the condensation products of glycols with alkylene oxides or dicarboxylic acids, respectively.
The polyurethane elastomers are formed by curing the urethane prepolymer. Curing is the reaction of the terminal isocyanate groups of the prepolymer with active hydrogens of a polyfunctional compound so as to form high polymers through chain extension and, in some cases, crosslinking. Diols, especially alkylene diols, are the most common curing agents for MDI-based urethane prepolymers, and representing such diols with the structure HO--X--OH, where X is an organic moiety, most usually an alkylene group, the resulting polymer has as its repeating unit, EQU (--Y--NHCO.sub.2 ZO.sub.2 CNH--Y--NHCO.sub.2 --X--O--CONH--)
Where a triol or a higher polyhydric alcohol is used crosslinking occurs to afford a nonlinear polymer.
Although other polyfunctional chemicals, especially diamines, are theoretically suitable, with but a few exceptions none have achieved commercial importance as a curing agent. The major exception is 4,4'-methylene-di-ortho-chloroaniline, usually referred to as MOCA, a curing agent which is both a chain extender and a crosslinker. TDI-based urethane prepolymers typically are cured with MOCA, and the resulting products account for perhaps most of the polyurethane elastomer market. One reason that polyhydric alcohols generally have gained acceptance as curing agents is that their reaction with urethane prepolymers is sufficiently fast to be convenient, but not so fast as to make it difficult to work with the resulting polymer. In casting polymers it is desirable that the set-up time be reasonably short, yet long enough for the material to be cast into molds. This property is conventionally referred to as pot life. Generally speaking, diamines react with prepolymers, and especially MDI-based prepolymers, so quickly that they are not usable as curing agents. However, primary aromatic diamines with electronegative groups on the aromatic ring, or with alkyl groups ortho to the amino moiety, exhibit sufficiently decreased reactivities with some prepolymers as to afford a desirable pot life, hence the use of, for example, MOCA as a curing agent for TDI-based urethane prepolymers. However, MOCA and other of the aforementioned diamines still remain too reactive to be used, for example, with MDI-based urethane prepolymers.
Until recently only primary aromatic diamines seem to have been used as curing agents. Presumably this is because secondary diamines were expected to have an unacceptably long pot life, and because they could act only as chain extenders with urethane prepolymers in contrast to the crosslinking capabilities of primary diamines. However, we have found that certain N,N'-dialkyl-4,4'-methylenedianilines alone are generally effective curing agents for a broad range of urethane prepolymers; see U.S. Pat. No. 4,578,446. The resulting polyurethanes often have the advantage of being thermoplastic rather than thermosetting, thereby making them especially useful as coatings, adhesives, and sealants. The secondary aromatic diamines of that invention have commercially acceptable pot lives as curing agents for many prepolymers, and afford products with an impressive variety of properties.
The thermoplastic properties referred to above result from the alkyl aromatic diamines as curing agents being exclusively chain extenders. It appeared to us that where thermoplasticity was undesirable, blends of the aforementioned diamines and polyhydric alcohols which would give some measure of crosslinking would afford elastomers with desirable properties. In fact, we have found that such blends afford polyurethanes with substantially increased tear resistance and tensile strength and significantly decreased compression set. The resulting polyurethanes are quite desirable articles of commerce, and the use of blends containing alkyl aromatic secondary diamines significantly extends the scope of the latter as curing agents.
Huffaker et al., U.S. Pat. No. 3,846,351, previously have disclosed the use of N,N'-dialkylphenylene diamines in admixture with polyols as combination catalysts and chain extending agents in the one-shot preparation of polyurethanes. In that method polyurethanes do not result from the reaction of a urethane prepolymer with an amine or polyol, but rather from the reaction of an isocyanate itself with a polyol containing some amine. This distinction is important because of the well known differences in polyurethane obtained via the one-shot and urethane prepolymer methods. Additionally, the patentees recommended a mixture containing a minor amount of amine, from 0.01 to 50 parts by weight per 100 parts polyol, and taught preferably a mixture containing 0.5-5 parts by weight amine per 100 parts by weight polyol. The patentee used such blends with a small amount of a blowing agent to prepare flexible polyurethane foams whose tensile strength and tear resistance were in the range of 8-25 psi and 1.5-3 pli, respectively. In stark contradistinction, the polyurethanes resulting from this invention have a tensile strength 50-900 and a tear resistance 40-400 times greater than the patentees' products.
Moreover, the blends of this invention require a major amount of amine relative to polyol. The relative amounts of these components is best expressed in terms of their equivalent weight, and in this context, the ratio of amine to polyol is at least 1:1, or 50 percent on the basis of equivalent weight. The optimum set of properties is often obtained when the mixture contains 20-35% polyol, or 65-80% amine.